This invention relates to a mass spectrometer and more particularly to a mass spectrometer provided with means for monitoring the level of degradation of its detector. The invention also relates to a method of monitoring the level of degradation of the detector of such a mass spectrometer.
In a mass spectrometer, a sample to be analyzed is initially ionized in an ionization chamber (herein referred to as the "ion source"). Ions of different kinds are normally generated in the ion source and are then accelerated by an ion lens to enter a mass filter comprising, for example, a quadrupole such that only the ions having a specified mass-to-charge ratio are allowed to pass through the filter and are detected by a detector.
Secondary electron multiplier tubes are the most commonly used type of detectors used with a mass spectrometer. A secondary electron multiplier tube is a detector adapted to output an electric signal with intensity according to the number of incident electrons by making use of a metal which emits a larger number of secondary electrons than the number of incident ions thereon with energy greater than a specified value. In general, members which are made of such a metal are arranged in a plurality of stages such that the number of secondary electrons will increase in a step-wise fashion and those secondary electrons emitted from the metal member of the last stage are taken out as the electric signal. A specified voltage difference is applied between each mutually adjacent pair of metal members when ions are being detected but the ion-electron multiplication factor (the ratio between the number of emitted electrons and that of the incident ions) will naturally change if this voltage is varied.
These metal members become degraded due to pollution by ions whenever a sample analysis is carried out, and this affects the ion-electron multiplication factor. One of the prior art methods for checking the level of degradation of a secondary electron multiplication tube has been to introduce a specified amount of a reference sample is introduced into the mass spectrometer while the applied voltage (to the metal members) is set at a specified level and to detect the ions generated from this reference sample as described above. By comparing the detected intensity of the output signal from the secondary electron multiplier tube with its initial value when the tube was still new, one can determine the condition of the degradation.
This method of judging the condition of degradation is not truly trustworthy. For example, although the condition of degradation is the same, the detected intensity of the output signal from the secondary electron multiplication tube will be lower if a different component of the mass spectrometer such as its ion source is degraded because this will be adversely affecting the efficiency of generating ions. In other words, one cannot determine with a method as described above if a drop in the intensity of the output signal is due to the degradation of the secondary electron multiplier tube itself or that of some other component. As a result, one may end up carrying out a wasteful maintenance work although the cause of the drop in the signal intensity is elsewhere by mistakenly believing that the cause was in the detector.